Wire saws are extensively used to slice silicon for solar and micro-electronics applications. The wire saws are also used for slicing a variety of other materials including sapphire, gallium arsenide (GaAs), indium phosphide (InP), silicon carbide (SiC), glass, lithium tantalate (LiTaO3) Z-cut crystals, lithium niobate (LiNbO3), lithium triborate (LiB3O5), quartz crystals, ceramics like aluminum nitride (ALN) and lead zirconate titanate (PZT), magnetic materials/parts, optical parts and the like material. The wire saws typically use a 120-180 micron diameter steel wire, which is several hundred kilometers long (FIG. 1). The wire is wound around a supply spool 110, a set of rollers called “wire guides” 130 to make a bed of parallel moving wire, often called “wire web” 140, and a take-up spool 120 as shown in FIG. 1. The wire guides 130 have equally spaced grooves on their outer surface to control spacing between the wires as it goes around the wire guides 130. The distance between the grooves, called pitch, eventually decides thickness of the wafers.
The work piece or the ingot 150, which needs to be sliced, is first glued to a plate 160 and then mounted on the wire saw. Then the ingot 150 is pressed with a vertical motion (top to bottom or bottom to top) against the horizontally moving wire web 140. The wire travels at a speed of about 15 meters/sec (or even higher) during slicing of wafers. Abrasive slurry, mainly made up of silicon carbide grains and a lubricant (e.g., polyethylene glycol or mineral oil), is introduced over the wire web 140. The abrasive slurry 210 coats the wire and travels to the cutting zone as shown in FIG. 2.
Also, it can be seen in FIG. 2 that, the abrasive slurry 210 tends to flow downwardly and away from the slicing zone, thereby significantly reducing the efficiency of slicing during the sawing operation. Further, it can be seen that a significant amount of abrasive slurry 210 is wasted by not being used in the slicing operation as the abrasive slurry 210 tends to flow downwardly and away from the cutting zone. Furthermore, it can be seen in FIG. 2 that, the abrasive slurry 210 flows perpendicular to direction of the horizontally moving wire web 140 (FIG. 2). Also, it can be seen in FIG. 2 that, majority of the abrasive slurry 210 does not pass through the ingot 150 and instead falls to the ground (bottom) of the wire saw. Further, in the conventional system using low viscosity slurries, risk of particles separating out of the abrasive slurry 210 is high.
Typically, slicing is achieved by slowly pushing the ingot 150 against the wire web 140. Furthermore, as cutting progresses, very fine silicon particles are loaded into the slurry. These particles in the slurry can increasingly adhere to the wafer surface as a function of time during the process. This is particularly true for very thin wafers, which require a much longer time to cut. Therefore, prompt cleaning is essential in all wire saw operations.
Slicing is completed when the ingot 150 completely passes through the wire web 140. At this point, the wafer stack which is held to the plate 160 is slowly pulled out of the wire web 140. After completing slicing and removing the stack of wafers from the wire saw the wafers are then cleaned immediately with water and other solvents to remove the abrasive slurry 210, otherwise the abrasive slurry 210 may stain the wafers thereby making them unusable in downstream processes. Further, the slurry remaining between the wafers needs to be removed quickly otherwise the slurry between the wafer can harden and hold the wafers together tightly and can make it difficult to remove and in some instances can break the wafers.
The current wire saws generate heat during slicing. Further, as the wafers become thinner, the cutting surface area increases significantly and as a result this can significantly increase the amount of heat generated during slicing. Also, the current wire saws cannot dissipate such heat generated during slicing. Further, lesser area is generally available for heat dissipation by radiation during slicing due to the slurry getting loaded between the wafers. This can lead to significant thermal stress in the wafers. Furthermore, the heat generated during slicing can soften the glue holding the stack of wafers to the plate 160. This can result in wafers dislodging from the plate 160 and breaking during slicing.
Further, as the silicon wafers are manufactured to thinner specifications, the sensitivity of these thinner wafers to any stress is significantly increased and these wafers can readily break. Currently, the standard for the solar industry is wafers sliced to a thickness of about 200 micrometers (microns; μm).